docs: def readVersion(self):
          """ Read the document version.
          ::
              <designspace format="3">
          """
          ds = self.root.findall("[@format]")[0]
          raw_format = ds.attrib['format']
          try:
              self.documentFormatVersion = int(raw_format)
          except ValueError:
              # as of fontTools >= 3.27 'format' is formatted as a float "4.0"
              self.documentFormatVersion = float(raw_format)